Terminal concentric bushing with current transformer



1957 E. c. WHITNEY 2,809,358

I TERMINAL CONCENTRIC BUSHING WITH CURRENT TRANSFORMER Filed Aug. 31, 1954 -","",HIUIUIIIIIIIIIIHIInnnnnniu- .-""n4",,.IIHIIllIllillillllllflll. I

I IIIIIIII/IIIIIIIIIIII/IIIIIIIIIII/II/I/I/l/I/I/I/III/IIIIII/II United States Patent TERMINAL CONCENTRIC BUSHING WITH CURRENT TRANSFORMER Eugene C. Whitney, Forest Hills, Pa, assigner to Westinghouse Electric (Importation, East Pittsburgh, Pa, a corporation of Pennsylvania Application August 31, 1954, Serial No. 453,353

'7 Claims. (Cl. 336-174) My invention relates to terminal concentric bushings, or concentric multiple-conductor lead-in bushings for electrical apparatus. While it is not altogether limited to any particular application, my invention is particularly applicable in a hermetically sealed bushing for highvoltage, high-current electrical apparatus which is enclosed in a hermetically sealed apparatus-housing, such as a transformer or a hydrogen-cooled synchronous condenser or three-phase generator.

Large three-phase machines have heretofore commonly had three individual phase-terminals and three separate ground-terminals, all of which were brought out of the machine through six terminal bushings. When large alternating currents are conducted through holes in the machine-housing, particularly in the steel plates of hydrogencooled machines, magnetic flux circulates around the cur rent-bushing and heats up the steel plate, to such an extent that it has been necessary, in times past, to place non-magnetic insert-plates in the portions of the steel plates surrounding the bushings, or, in extreme cases, to use entire plates made of non-magnetic material, in the place where the bushings come out of the machine, which has frequently been a nuisance. in high-voltage machines, such as machines of from 6 to 30 kilovolts, a difficult design-problem has frequently been introduced by the necessity for providing sufficient spacing between the six previously used bushings, in order to provide sufiicient clearance between terminals, to withstand the voltage involved. In heavy-current machines, in which each lead carries well over 2060 amperes at 60 cycles, serious problems have been encountered, because the magnetic field surrounding one bushing would induce eddy currents in the lead-in terminahconductors which are in the adjacent bushing.

My present invention involves the use of concentric bushings on high-voltage electrical machines or apparatus. These concentric bushings may be either three-phase, with three concentric lead-in conductors in the bushings, or they may be concentric two-conductor lead-in conductor-bushings in which the two conductors of each bushing consist, respectively, of the terminal or phaseconductor and the ground-conductor of one phase of the machine, so that the currents which are carried by the two conductors are normally equal and in phaseopposition to each other, so that the concentric bushing is surrounded by essentially zero flux, which avoids excessive heating in the housing-plates surrounding the bushings, and also avoids the objectionable induction of eddy currents in the conductors of the adjacent bushings.

An important feature of my invention is a practical design which makes the bushing gas-tight or hermetically sealed, while still taking care of the thermal-expansion problem whereby parts of ditferent materials have different expansion-rates. My design also places two hermetically sealing gaskets in series with each other, for preventing the escape of hydrogen from the machinehousing. My invention also makes it possible to till the ice J bushing with either dry or a flowed-in insulation. My invention also provides practical bushing-assembly means, whereby the bushing can be assembled without damaging the gaskets, after which the assembly-means can be released to put pressure on the gaskets.

My invention, particularly in its two-conductor embodiment, further provides a means for providing a single bushing-type or through-conductor type of current-transformer which is mounted on the central insulating housing-member of the concentric bushing, preferably on the outside of the machine-housing, as an improved means for providing the necessary differential relay protection to detect internal faults within the machine. Heretofore, a separate current-transformer has been provided for each of the two terminals of each phase of the machine, these cin'rent-transformcrs being connected in opposition to each other, to energize a difierentially responsive relay, thus resulting in inaccuracies due to different coresaturations of the two transformers, on either A. C. or D. C. transients, and other difficulties, besides involving a considerable amount of secondary current-transformer wiring, and difficulties in the structural design and in obtaining an adequate usable sensitivity of the differential relay. Since my two-conductor concentric bushing normally has essentially zero flux surrounding it, a through-current transformer, surrounding the bushing, normally develops no secondary voltage during faultfree operating-conditions of the machine, but such a transformer sensitively responds to any internal faultcondition in the machine, which would cause the two currents to be no longer both equal and in phase-opposition. The possibility of placing such a through-current transformer on the part of the concentric bushing which extends outside of the machine-housing avoids the necessity for bringing out secondary leads from the interior of the machine, in order to energize the simple overcurrent relay which is all that is now necessary to serve as a difierential relay.

With the foregoing and other objects in view, my invention consists in the circuits, systems, combinations, structures, parts, and methods of design and operation, hereinafter described, and illustrated in the accompanying drawing, wherein:

Figure 1 is a sectional view, not to scale, illustrating the general principles of my invention in a two-conductor form of concentric-bushing design;

Fig. 2 is a very diagrammatically shown illustrative three-conductor form of embodiment of my invention, only the right-hand half of the bushing being shown, and certain of the less-important or less-essential features being omitted; and

Fig. 3 is a diagrammatic view of the lower portion of Fig. 2, embodying certain structural modifications which are shown for illustrative purposes.

in Fig. l, i have shown my invention as being applied to an alternating-current apparatus a, of which ony a fragmentary portion is illustrated, enough to show that it has two opposite-potential leads 5 and :5, which normally carry substantially equal currents in substantially exact phaseopposition during the fault-free operation of the apparatus, as previously mentioned. This apparatus 4 is illustrated as being provided with an apparatus-housing 7, which, by way of giving a practical example, is shown as being filled with hydrogen Hz, which is usually at a pressure higher than the surrounding atmosphere or air. The housing 7 is provided with a lead-receiving hole or opening 8 to accommodate the passage of the two leads 9 and 10 through the frame or housing 7 of the machine or apparatus. To prevent the escape of hydrogen through this lead-receiving opening 8, the is provided with a lead-box 12, which surrounds said opening, and which serves in'eflect as a part or the gas-tight housing 7 of the machine. The lead-box 12 is provided with a bushingreceiving hole 13, for receiving a hermetically sealed concentric two-conductor lead-in bushing 14 which is constructed in accordance with my invention.

My concen t ric two-conductor lead-in bushing 14-, as shown in Fig. 1, comprises an inner lead-in conductor 15, and an electrically insulated outer tubular lead-in conductor 16, in substantially concentric spaced relation to each other, the inner conductor 15 having the greater length and the smaller cross-sectional or transverse dimension, said inner conductor having projecting ends 15a and 15b which extend beyond the ends of the outer conductor 16. In cases where the bushing 14 is to be hermetically sealed, so as to maintain the gas-tightness of the housing 7'12 through which it is placed, as shown in Fig. 1, it is neces sary thatthe inner conductor 15 shall be hermetically closed, either by being solid, or, if it is tubular in shape, by having a hermetically connected cross-piece or closurepiece inside of it, preferably at the end of the lower end 15b which extends outside of the apparatus-housing, as shown at 17, or by any other means which maintains the integrity of the gas-tight enclosure for the machine or apparatus It is understood that the inner conductor 15 may advantageously be ventilated or cooled, in any desired manner.

Each of the concentric conductors 15 and 16 has a rigid flanged end, shown as being the top end in each case, comprising a rigid laterally extending annular electrically connected terminal-connection flange 15c and 160, as the case may be. These flanges are perforated for accommodating a bolted connection to the machine-leads 9 and 19. Preferably, each flange 15c and 16c is permanently and hermetically connected to its conductor 15 or 16, as the case may be.

The bushing-assembly 14 is provided, preferably at its lower end which extends outside of the machine-housing 712, with a preferably removable, rigid flanged, terminal-connection cap-member 18, which is electrically con nected, in any suitable manner, to the lower or unflanged end of the inner lead-in conductor 15. Preferably, this electrical connection is a removable screwed-on electrical connection 18a, which may, or may not, be hermetically tight. The cap-niember 13 is provided with an integral laterally extending rigid annular terminal-connection flange-portion 18b, which is adapted to receive one of the outside terminal-leads (not shown), outside of the machine or apparatus. In cases where the bushing 14 is required to be hermetically sealed, the cap-member 18 should also have a hermetically closed central end-portion 180, unless the screwed-on connection 18a is hermetically tight.

The entire length of the inner lead-in conductor 15 is preferably encased in a tubular insulating sheath 19, which may be formed of suitably impregnated tape, and which is surrounded by the tubular outer conductor 16.

The lower end of the outer tubular lead-in conductor 16 is provided with a rigid laterally extending annular terminal-connection flange-member 22, which is electrically connected to said lead-in conductor 16, preferably having a removable screwed-n connection 22a.

Surrounding the tubular outer lead-in conductor 16, there is provided a central tubular porcelain or other rigid insulating housing-member 23, preferably with an annular space 24 between this housing-member and the outer lead 16. The two projecting ends a and 15b of the inner lead-in conductor 15 are each surrounded by a separate auxiliary porcelain or other insulating housing-member 23a or 23]), as the case may be, preferably with annular spaces 24a and 24b between said housing-members and the inner conductor 15.

Each of the three housing-members 23a, 23 and 23b must be provided with a separate sealing-means 23a, 23a", 23', 23", 23b and 2315, respectively, at each end, for sealing its end, preferably hermetically, to the adjacent terminal-connection flange 15c, 16c, 22 and 1%,

as the case may be. In order that the parts may be initially assembled, it' is necessary that the sealing-means at at least one end of each of the three insulating housing-members 23a, 23 and 23b shall be completed or perfected after the parts of the bushing have been assembled. These after-assembly seals may conveniently include, at at least one end of each housing-member, one or more removable compressible sealing-gaskets 25 which require a predetermined endwise pressure to maintain their hermetic sealing-qualities. The sealing-means at the other end of each of these three housing-members could be of either a permanent type or a removable. type: for convenience, preferably all six of the sealing-means 23a, 23a", 23', 23", 23b and 23b are of the compressible-gasket type, as shown.

Because of the need for taking care of unequal thermal expansions and contractions, coupled with the need for maintaining the requisite endwise pressure on the compressible sealing-gaskets 25, it is necessary that at least some one or more of the sealing-means shall be, or include, an axially expansible, spring-pressed, hermetic sealingelement, as shown at 26. These spring-pressed elements 26 may take different forms.

In the form shown in Fig. 1, the spring-pressed sealingelement 26 is associated with the removable flange-member 22 which is screwed onto the lower end of the outer conductor 16. This spring-pressed sealing-element 26, con sists of a hub 27, to which is brazed the flange 22, making a gas-tight joint. Disposed respectively above and below the flange 22, are two spring-pressed flanges 27a and 271'}, which engage the gasket-seals 25, respectively, for the lower end of the central insulating housing 23 and the upper end of the lower insulating housing 23b. These spring-pressed flanges 27a and 271) are pressed apart by a plurality of compression-springs 27c having their ends pressed against the respective spring-pressed flanges. The spring-pressed sealing-element or assembly 26 also includes two bellows 28a and 28b, or other axially expansible hermetic sealing-means, for sealing the respective spring-pressed flanges 27a and 27b to the flangemember 22 at a point inside of the compression-springs 270.

As shown in Fig. 1, it is usually very desirable to provide this spring-pressed sealing-assembly 26 with a means for temporarily holding the spring-pressed flanges 27a and 27b closer together than normal, as by means of bolts 29, which may be tightened up against abutmentsleeves 30, so as to remove the spring-pressure during the assembly-operation, thus avoiding the occurrence of a damaging friction on the compressible sealing-gaskets 25 during the screwing-on operations of the removable flangemember 22 and the removable cap-rnember 18, respectively. It will be understood that, after the bushing has been completely assembled, the spring-tightening bolts 29 may be loosened or removed, thus applying the necessary seal-maintaining pressures on the sealing-gaskets 25.

The annular spaces 24a, 24b and 24b, inside of the insulating housing-members 23a, 23 and 23b, respectively, could be left filled with air or hydrogen or other gas, but preferably these spaces are filled with a flowed-in insulating filling-means 32, which may be oil, or other insulating filling-compound which is capable of being flowed in, at least 'during the filling operation, whether or not said compound hardens or remains liquid or semi-liquid thereafter. Preferably, a small-air or gas-space 32b is left at the top, for thermal expansions or contractions. While it is possible that'each of the annular spaces 24a, 24 and 24b may be hermetically sealed off to itself, it is advantageous that all three of these spaces shall be in communication with each other, as shown in Fig. 1, so that a single fillingoperation will 'fill all three of the spaces with the oil or other insulating compound 32.

The bushing 14 must necessarily have some means for mounting it within the bushing-receiving hole 13 of the lead-box 12 or other portion of the apparatus-housing 7.

5 As shown, an intermediate part of the central insulating housing-member 23 of the bushing 14 in Fig. l is provided with a hermetically and mechanically tight sleeve 33, which carries a hermetically and mechanically tight mounting-flange 34, which is assembled against the leadbox 12 with an intervening sealing-gasket 35.

As previously intimated, my two-conductor concentric bushing 14, as shown in Fig. 1, lends itself admirably to the use of a bushing-type or through-conductor type of current transformer 36 which is mounted on the central insulating housing-member 23, preferably on the air-side or outside of the apparatus-housing 7-12, as shown in Fig. 1.

My invention is not limited to a concentric-type lead-in bushing which has only two conductors, as shown in Fig. 1. It may have three such conductors, or any other de sired number of conductors.

Thus, as shown in Fig. 2, a portion of a three-conductor concentric bushing is diagrammatically shown, comprising an innermost conductor 41, and two electrically insulated outer tubular conductors 42 and 4-3 of graduated lengths, in substantially concentric spaced relation to each other, the innermost conductor 41 having the greatest length and least cross-sectional or transverse dimension, while the outermost conductor 43 has the smallest length and the greatest cross-sectional dimension. The intermediate conductor 42 has projecting ends 42a and 42b which extend beyond the ends of the outermost conductor 43, while the innermost conductor '41 has projecting ends 41a and 41b which project beyond the ends of the intermediate conductor 42. Each of the three conductors has an integral terminal-connection flange 41c, 42c and 430, as the case may be, as will be understood from the description of the Fig. l embodiment.

As shown in Fig. 2, the lower end of the outermost conductor 43 is provided with a double-pressure springassembly 45, which is similar to the element 26 in Fig. 1, except that the terminal connection may be made to one of the spring-pressed flanges 47b, instead of to the removable flangemember 47. The connection between the flange 47 and the lower end of the outermost tube 43 may be either removable or a more permanent connection, such as a brazed connection. It is to be understood that these remarks apply also to the Fig. 1 construction. As also shown in Fig. 2, the lower end of the intermediate tube 42 is similarly provided with a double-pressure springassembly 56, which is carried by a flange 57 which is suitably connected to the conductor 42. The central or innermost conductor 41, in Fig. 2, may be closed by a cap-member 13, which may be similar to that which is shown in Fig. 1.

The three-conductor bushing of Fig. 2 is provided with live porcelains or insulating housing-members, namely a central housing-member 43, which may be similar to the member 23 of Fig. l, and four separate auxiliary insulating housing-members 41a, 42a, 42b and 41b, which may be similar to the members 23:: and 23b of Fig. 1.

In both Figs. 1 and 2, the axial pressure which is applied to the various sealing-gaskets is maintained by means of the compression-springs of a double-pressure springassembly 26 (Fig. 1) or assemblies 46 and 56 (Fig. 2). in either case, instead of having one double-pressure spring-means for pressing both ways against the two insulating housingmembers between which that springassembly is placed, it is possible to provide a single-action spring-means which is associated with the bottom or unflanged end of each of the three conductors 41, 4-2 and 43, as shown at 61, 6?. and 63 in Fig. 3. Each of these single-way spring-assemblies comprises a terminal-connection plate 6 a spring-pressed plate 65, a plurality of compression-springs 66, and a bellows or other axially expansible hermetic seal 67, as will be readily understood.

In all forms of embodiment of my invention, it will be noted that the final tightening-movement of the bushlug-assembly, as when the bottom end-cap is pushed or screwed up into place, and held there, holds the various compression-springs of the sealing-assemblies under suflicient compression to apply the necessary axial pressure which is required by all of the sealing gaskets which are disposed at the ends of the various insulating housingmembers of the assembly.

in all of the illustrated forms of embodiment of my invention, when I refer to certain flanges as being terminal-connection flanges, I mean that the terminal connections may be made either directly to these flanges or to other metal parts which are electrically connected therewith so as to be electrically at the same potential as said flanges.

While I have illustrated my invention in only three different forms of embodiment, I wish it to be understood that my invention is not limited to these illustrated forms, and that it includes various substitutions of equivalents, or omissions or additions of parts, without departing from the essential spirit of the invention.

l claim as my invention:

1. A concentric multiple-conductor lead-in bushing for an electrical apparatus, comprising: an innermost leadin conductor and one or more electrically insulated outer tubular lead-in conductors of graduated lengths in substantially concentric spaced relation to each other, the innermost conductor having the greatest length and the least transverse dimension, while the outermost conductor has the smallest length and the greatest transverse dimension, each of the inner conductors having projecting ends which extend beyond the respective ends of the next shorter outer conductor; the innermost conductor being closed; each of said conductors having a first end comprising a laterally extending rigid annular electrically connected terminal-connection flange; a rigid flanged terminal-connection cap-member having an electrical connection to the second end of the innermost lead-in conductor, said cap-'nember having a laterally extending rigid annular terminal-connection flange-portion; a laterally extending rigid annular terminal-connection flange-member for each of the outer tubular lead-in conductors, each of said flange-members having an electrical connection to the second end of its lead-in conductor; a central rigid tubular insulating housing-member surrounding the outermost lead-in conductor; a separate auxiliary rigid tubular insulating housing-member surrounding each projecting end of successive inner lead-in conductors; and a separate sealing-means at each end of each of the rigid tubular insulating housing-members, for sealing its end of the housing-member to the adjacent terminal-connection flange of one of the lead-in conductors; the sealing-means at at least one end of each of said housing-members including a compressible sealing-gasket; and at least one of the two sealing-means at the two ends of the central housing-member, and at least one of the two sealingmeans at the two ends of one of the two auxiliary housing-members which surround the two projecting ends of each of the successive inner lead-in conductors, including an axially expansible, spring-pressed, hermetic sealingelement.

2. The invention as defined in claim 1, characterized by: a through-conductor type of current-transformer mounted on the central insulating housing-member.

3. A concentric multiple-conductor lead-in bushing for an electrical apparatus, comprising: an innermost leadin conductor and one or more electrically insulated outer tubular lead-in conductors of graduated lengths in substantially concentric spaced relation to each other, the innermost conductor having the greatest length and the least transverse dimension, while the outermost conductor has the smallest length and the greatest transverse dimension, each of the inner conductors having projecting ends which extend beyond the respective ends of the next shorter outer conductor; the innermost conductor being closed; each of said conductors having a first end comprising a laterally extending rigid annular electrically connected terminal-connection flange; a rigid flanged terminal-connection cap-member having an electrical connection to the second endv of the innermost lead-in conduc tor, said cap-member having a laterally extending rigid annular terminal-connection flange-portion; a laterally extending rigid annular terminal-connection flange-member for each of the outer tubular lead-in conductors, each of said flange-members having an electrical connection to the second end of its lead-in conductor; a central rigid tubular insulating housing-member surrounding the outermost lead-in conductor; a separate auxiliary rigid tubular insulating housing-member surrounding each projecting end of successive inner lead-in conductors; and a separate sealing-means at each end of each of the rigid tubular insulating housing-members, for sealing its end of the housing-member to the adjacent terminal-connection flange of one of the lead-in conductors; the sealing-means at at least one end of each of said housing-members including a compressible sealing-gasket; and the sealingmeans at each of the terminal-connection flange-members being a double-pressure assembly having a spring-pressed flange on each side of the terminal-connection flangemember, for energizing the adjacent end of one of the insulating housing-members, the said double-pressure assembly having a plurality of compression-springs having their ends pressing against the respective spring-pressed flanges of that assembly, and each assembly further having an axially expansible sealing-means for sealing each spring-pressed flange to the terminal-connection flangemember at a point inside of the compression springs.

4. A hermetically sealed concentric multiple-conductor lead-in bushing for an electrical apparatus, comprising: an innermost lead-in conductor and one or more electrically insulated outer tubular lead-in conductors of graduated lengths in substantially concentric spaced relation to each other, the innermost conductor having the greatest length and the least transverse dimension, While the outermost conductor has the smallest length and the greatest transverse dimension, each of the inner conductors ha ing projecting ends which extend beyond the respective ends of the next shorter outer conductor; the innermost conductor being hermetically closed; each of said conductors having a rigid flanged end comprising a permanent laterally extending rigid annular hermetically and electrically connected terminal-connection flange; a removable rigid flanged terminal-connection cap-member having a removable electrical connection to the unflanged end of the innermost lead-in conductor, said cap-member having a hermetically closed central endportion and a laterally extending rigid annular terminalconnection flange-portion; a separate removable laterally extending rigid annular terminal-connection flange-merr her for each of the outer tubular lead-in conductors, each of said removable flangemembers having a removable electrical connection to the unflanged end of its lead-in conductor; a central rigid tubular insulating housing-member surrounding the outermost lead-in conductor; a separate auxiliary rigid tubular insulating housing-member surrounding each projecting end of successive inner leadin conductors; and a separate sealing-means at each end of each of the rigid tubular insulating housing-members, for hermetically sealing its end of the housing-member to the adjacent terminal-connection flange of one of the lead-in conductors; the sealing-means at at least one end of each of said housing-members being a removable seal including a compressible sealing-gasket which requires a predetermined endwise pressure to maintain its hermetic sealing-qualities; and at least one of the two sealing-means at the two ends of the central housing member, and at least one of the two sealing-means at the two ends of one of the two auxiliary housing-members which surround the two projecting ends of each of the successive inner lead-in conductors, being a spring-pressed assembly including an axially expansible, spring-pressed,

hermetic sealing element for maintaining the requisite endwise pressure on the compressible sealing-gaskets.

5. A hermetically sealed concentric multiple-conductor lead-in bushing for an electrical apparatus, comprising: an innermost lead-in conductor and one or more electrically insulated outer tubular lead-in conductors of: graduated lengths in substantially concentric spaced relation to each other, the innermost conductor having the greatest length and the least transverse dimension, while the outermost conductor has the smallest length and the greatest transverse dimension, each of the inner conductors having projecting ends which extend beyond the respective ends of the next shorter outer conductor; the innermost conductor being hermetically closed; each of conductors having a rigid flanged end comprising a pclintoieut rigid laterally extending annular hermetically and electrically connected terminal-connection flange; a removable rigid flanged terminal-connection cap-member having a screwed-on removable electrical connection to the unllanged end of the innermost lead-in conductor, said cap-member having a hermetically closed ccntral end-portion and a laterally cxtencing rigid annular terminal-conncction flange-portion; a separate removable laterally extending rigid annular terminal-connection fiangcmember for each of the outer tubular lead-in conductors, each of said removable flange-members having a screwed on removable electrical connection to the unfianged end of its lcad-in conductor; a central rigid tubular insulating housingnnember surrounding the outermost lead-in conductor; a sen auxiliary rigid tubular insulating housing-member stnrounding each projecting end of successive inner lend-in conductors; a separate sealing-means at each end of each of the rigid tubular insulating housingmembers, for hermetically sealing its end of the housingmembcr to the adjacent terminal-connection flange 01: one of the lead-in conductors; the sealing-means at at least one end of each of said housing-members being a removable seal including a compressible sealinggasket which requires a predetermined endwise pressure to maintain its hermetic sealing-qualities; at least one of the two sealing-means at the two ends of the central housingmember, and at least one of the two sealing-means at the two ends of one of the two auxiliary housing-members which surround the two projecting ends of each of the successive inner lead-in conductors, being a spring-pressed assembly including an axially expansible, spring-pressed, hermetic sealing element for maintaining the requisite endwisc pressure on the compressible sealing-gaskets; and a spring-compressing means for each of the spring-pressed assemblies, for holding the spring-means compressed to avoid the occurrence of a damaging friction on the compressible scaling-gaskets during the screwing-on operations of the removable flange-members and the removable cap-member.

6. An alternating-currcnt apparatus having two opposite-potential leads which normally carry substantially equal currents in substantially exact phase-opposition during the fault-free operation of the apparatus, said apparatus also having an apparatus-housing having a bushing-receiving hole therein, and a hermetically sealed concentric two-conductor lead-in bushing, mounted in said bushing-receiving hole, for said two 0pposite-potential leads of said apparatus; said bushing comprising: an inner lead-in conductor and an electrically insulated outer tubular lead-in conductor in substantially concentric spaced relation to each other, the inner conductor having the greater length and the smaller transverse diiension, the inner conductor having projecting ends which extend beyond the ends of the outer conductor; the inner conductor being closed; each of said conductors having a first end comprising a rigid laterally extending annular electrically connected terminal-connection flange; a rigid flanged terminal-connection cap-member having an electrical connection to the second end of the inner lead-in conductor, said cap-member having a laterally extending rigid annular terminal-connection flange-portion; a laterally extending rigid annular terminal-connection flange-member having an electrical connection to the second end of the outer lead-in conductor; a central rigid tubular insulating housing-member surrounding the outer lead-in conductor; a separate auxiliary rigid tubular insulating housing-member surrounding each projecting end of the inner lead-in conductor; a separate sealing-means at each end of each of the rigid tubular insulating housing-members, for sealing its end of the housing-member to the adjacent terminal-connection flange of one of the lead-in conductors; the sealing-means at at least one end of each of said housing-members including a compressible sealing-gasket; at least one of the two sealing-means at the two ends of the central housing-member, and at least one of the two sealing-means at the two ends of one of the two auxiliary housing-members which surround the two projecting ends of the inner lead-in conductor, including an axially expansible, springpressed, hermetic sealing element; and a separate laterally extending annular mounting-flange disposed at an intermediate point along the length of the central housing- 10 member, said mounting-flange being sealed to said central housing-member.

7. The invention as defined in claim 6, characterized by: a through-conductor type of current-transformer mounted on the central insulating housing-member for responding to faulty operating-conditions in which the currents in the two opposite-potential leads of the apparatus are not both substantially equal and in substantially exact phase-opposition.

References Cited in the file of this patent UNITED STATES PATENTS 1,539,658 Fitz Gerald May 26, 1925 2,135,321 Brandt Nov. 1, 1938 2,411,656 Henderson NOV. 26, 1946 2,640,102 Woods May 26, 1953 OTHER REFERENCES Article: New Transformers Over Generator Bushings, pages 98 and 99, Westinghouse Engineer, May 1951. 

